High quality AC turntable motor

Just a thought, but have you tried a new belt and a good cleaning of the motor pulley and subplatter belt contact area? The stylus drag you are hearing sounds like compliance being introduced through belt slip or stretch.

If you can find one, I've found the Berger-Lahr motors to be considerably better than the Crouzet, Hurst or Saia, at least in terms of cogging. Unfortunately, the Airpax/Premotec motors are rather difficult to source however they offer the smoothest cogging characteristics out of any of them. Then again, I use a Mark Kelly Synchrotron controller, so I can null out most motors so that cogging vibration isn't too much of an issue.

Good luck and keep us posted.

Interesting to refer to motor mechanics in terms of digital distortion. The "pulsing" you refer to is known as cogging, which refers to the rotation of the rotor through the fixed poles (stator) on a synchronous motor. Depending on the rotational speed of the motor, the number of poles varies; a 300 RPM motor will generally have 24 poles, while at 600 RPM, there will be twelve poles. The overall torque of the motor is determined by the field strength of the magnets; stronger magnets have more cogging and hence more torque.

I find that with most 115VAC synchronous motors, run at their approximately nominal frequency (50/60Hz), it's usually better to smooth out the cogging by running the motor at approximately 65-75% of its rated voltage. For example, I run my 600 RPM/115VAC motor at 55Hz for 33RPM with a voltage around 75VAC on one phase, and around 69VAC on the other. This optimizes the torque to cogging ratio such that the motor doesn't introduce vibration through the drive chain, yet maintains enough torque to overcome all stylus drag during heavily modulated passages.

The SDS is a decent controller, but its lack of parametric adjustment (phase angle, harmonic distortion, and individual phase amplitude) makes it nothing more than a glorified speed controller (though nice for a VPI owner to not have to swap pulleys). It does little, however, to address the differences found even in sample to sample runs of the same exact motor. Without being able to optimize every parameter of the motor, you can never really null out cogging, nor get the best performance out of any given motor.

I use Mark Kelly's Synchrotron AC-1 controller, which contains all of the aforementioned features (and then some) but has long since been unobtainable; this was a DIY Audio kit project from several years back. You bought the raw boards, bought the parts, understood the design's theory and you were basically on your own; you built the unit from the chassis on up - not a build for the novice. I admit, I've been spoiled by the controller's abilities.

Another possible candidate in the making is this gentleman's controller, based around a PIC/Atmal chip:

http://www.soundbound24.blogspot.ca/ (scroll down to the Bi-Onda controller). He seems to be on a hiatus, but I'm hoping he continues on the fascinating path he's on; his is the most promising and versatile design I've seen in a while.

I would gather that without upping the power handling of the output amplifiers, both this controller and my Kelly can't drive anything larger than say, 7 watts (the average synchronous motor in a TT is usually 3-5 watts).

There is also the Sander's speed controller over on Lenco Heaven: http://www.lencoheaven.net/forum/index.php?topic=2042.0 This appears to have applicability toward higher wattage motors (15-20 watts), and shaded-pole induction types as used in the Lenco, Garrard and others. I haven't gone through the entire post to tease out the details of the circuit, but it appears to cover the essential parameters. I'm still exploring the salient characteristics in this lengthy thread.

I've never had a Walker controller open to see what makes it tick, but I'm guessing it's about the same as the SDS. While a Linn Lingo has a nice sine wave synthesizer, it also doesn't allow for any parametric adjustment - despite the fact they're loaded with (unlabeled) trim pots.

While I advocate complete parametric motor adjustment, it's also a catch-22. You need some test gear like a multimeter and an oscilloscope to really maximize performance; a little too far this way or that with some settings and you can really mess things up (as in your motor stalls, vibrates and you become sad because your table no longer spins). This, I believe, is the reason nobody has a desire to market a motor controller which contains all the necessary parametric adjustments: the support issues would instantly turn into a complete nightmare since most folks simply aren't technically savvy enough reap the benefits or dig themselves out of the inevitable holes that would result.

Win,

But VPI use Hurst motors!! ;-))

Redglobe,

From a cursory glance, I would say that the Sound Carrier unit is very similar to either the Walker or the SDS (with the added ability to trim output voltage). Hence, no - I don't believe the unit to offer full parametric adjustment, far from it in fact. The Heed Orbit would fall into this same category as well.

To give you a better idea, my Kelly controller has approximately seven trim pots that can be adjusted to not so subtle effect. Of course, this excludes the speed selector switch, frequency setting 'switches' and the on/off switch. I do not, however, have a fancy meter on the front of my controller ;-)

Grbluen2,

I'm afraid I can't be more specific about building a parametric two phase controller from the ground up. The easiest way out would be to use two MP3 players with continuous sinusoidal waveform loops (of differing frequency for speed selection) fed through a cheap (maybe class D) amplifier such that one could vary the output amplitude (voltage) for each phase (IMO, this is the most important parameter to adjust). While this doesn't give one full parametric control, it is essentially the idea behind the first controller I mentioned (http://www.soundbound24.blogspot.ca/). It uses a chip which contains the command set for generating sine waves of varying frequency (speed), lagging one phase behind the other, changing amplitude and (were the builder to incorporate it) varying harmonic distortion components. In all, he's on the right path. The devil of course is in the details.

Grbluen2,

There is no procedure for finding the 'magic setting', as every motor is different. For example, I have three Hurst 300 RPM motors, but really only one is worth a damn - the other two can never be quieted down to a suitable level, and never quite sound right. I set a motor so as to null vibration as much as possible - you can either hold the motor in your hand, use a mechanics stethoscope, or listen to the table rotating in a lead-out or locked groove. From there, I'll look at the waveform on an oscilloscope to ensure the sine waves look nice; I generally find that a slightly distorted waveform actually yields a better result than a pure sine wave (I don't know why). From there on, it's all in the listening, trying to balance torque requirements with vibration considerations. I'll vary the voltage of each phase slightly, and make assessments from there. For example, a motor with too much voltage will always place too much emphasis on the attack of notes, while too little will yield a bloated bass and a really laid back presentation. The more you play around, the easier it becomes to hear where you're at and establish a baseline sonic signature - speed never varies given the nature of the motor, but the way the motor deals with heavy modulation, stylus drag and the like can all be readily discerned. Unfortunately, there is no purely objective way of measuring any of this, like so much in this odd little hobby. Trust your ears, and beyond all else, have some fun.

You can't, as they are now unobtainable, nor do I suspect most owners would want to part with them. BTW, the Synchrotron runs off of 12 volts DC, and outputs either 115VAC or 230VAC.